904 Flex Plate for '72 340

Summit lists B&M 10239:

http://www.summitracing.com/parts/bmm-10239/overview/make/dodge

and

TCI 102390

http://www.summitracing.com/parts/tci-102390/overview/make/dodge

as being for external balance and 904.

I have no experience with those flexplates, or a 904 for that matter, so I can't say for sure, but might be worth looking into.

EDIT: No, they're not necessarily cost-effective options, but you didn't say if you needed SFI-approved or whatever, so... might be worth checking out anyway.

EDIT 2: Also, you cannot go by Summit's pictures, at least not for their flexplates, so don't go by the visual. I know this from experience. I recently ordered a $40 TCI one, a $60 Hughes one, and a $10 ATP one for 727 internal balance. They all looked totally different on their visual, all had different pricing obviously, but were all 100% identical in every way once they arrived. Two of them were just overpriced re-labelled stock flexplates.